Determination of distances to stationary or moving objects is an important measurement challenge encountered in many fields of science and technology. In some cases, the distances to the objects of interest are macroscopic and can be expressed in kilometers or larger units. This is true, for example, in determining distances or ranging remote structures or moving objects, such as vehicles. In other cases, the distances to the objects of interest are microscopic and can be expressed in millimeters or smaller units. Such conditions are encountered, for example, when determining distances between micro-structures on a silicon wafer. The prior art teaches a great variety of techniques to measure distances over various ranges in numerous fields and applications, including robotics and machine vision. An overview of a number of these techniques is found in “Where am I” Systems and Technologies for Mobile Robot Positioning, J. Borenstein, H. R. Everett, and L. Feng, A. K. Peters, Ltd., University of Michigan for the Oak Ridge National Lab (ORNL) D&D Program, Published by Wellesley, Mass., copyright April 1996.
In the present case, we are interested in determining distances that fall between the macroscopic and microscopic, e.g., distances on the order of a few centimeters or meters. More specifically, of particular interest are techniques that use optical beams and can perform accurate distance measurements in this intermediate distance range.
One of the approaches taught by the prior art is based on optical ranging cameras or range-finding camera systems. Some examples of such cameras and systems are described in U.S. Pat. Nos. 6,057,909; 6,034,716; 5,200,793 and by S. Christie, et al., in Measurement Science and Technology 6, September 1995, pp. 1301–1308. These systems are too cumbersome when a distance to one or just a few particular points or objects needs to be measured and no image of the scene is required.
Another approach taught by the prior art is based on scanning systems that use beams to determine distance. Most of these systems use the time-of-flight or propagation delay time to derive distance to the object. Several examples of corresponding apparatus and methods are found in U.S. Pat. Nos. 6,710,859; 6,064,471; 6,057,910; 5,959,734; 5,831,717; 5,724,123; 5,648,852 and 5,477,461. More sophisticated approaches using scanning are discussed in greater detail in a paper by Johnny Park, et al., “Dual-Beam Structured Light Scanning for 3-D Object Modeling”, Third International Conference on 3-D Imaging and Modeling, Proceedings May 28, 2001–Jun. 1, 2001, pp. 65–72.
Unfortunately, most of the prior art approaches using scanning beams are not suitable for use in simple and low-cost systems for accurate determination of medium-range distances to stationary or moving objects. Specifically, many of these techniques, including time-of-flight or propagation time delay, are not suitable or not sufficiently accurate for measuring distances in the intermediate distance range.